The present invention relates to transillumination imaging techniques used in medical or other applications for obtaining images of interior details in a living body or of other objects, and is more particularly directed to imaging techniques using infrared or near-infrared radiation.
Optical transillumination imaging is an imaging technique that shows promise as an alternative to x-rays, magnetic resonance, ultrasound, positron emission, thermal emission, or other techniques for forming images of certain internal details of a living body. The body portion or object to be imaged is illuminated with a source of radiation in the visible, infrared, or near-infrared range, and the body portion or object selectively absorbs, scatters and transmits the radiation to form an image that is recorded by a suitable detecting apparatus. Differences in absorption or scattering characteristics of the object under examination may lead to discernable features in the recorded image. For example, transillumination of tissue may provide a useful diagnostic modality for malignant tumors, which often exhibit greater attenuation or different scattering of light compared with neighboring healthy tissue, possibly due to a plentiful supply of blood and high concentration of mitochondria in the tumor.
Optical transillumination is potentially highly attractive in that it is noninvasive and nondestructive (the radiation is non-ionizing), optical radiation can be tolerated in comparatively large doses by the patient, it poses no radiation danger to the attending personnel, and it can reveal features not readily discernable by other techniques. In comparison with x-rays, for example, infrared or near-infrared radiation is more sensitive to soft tissue variations and gives appreciably better contrast. X-rays, however, provide extremely good spatial resolution (down to 100 microns), but at the sacrifice of good contrast.
Although infrared and near-infrared transillumination provides good contrast, it gives poor resolution, resulting in blurred images, in great part because of scattering. At infrared and near-infrared wavelengths, the radiation is strongly scattered as it passes through the tissue and surrounding medium. Radiation in this spectral range typically undergoes multiple scattering events before it emerges from the sample. The scattering events effectively smear out the spatial resolution.
Past efforts to improve infrared imaging have included improvements in infrared sources for transillumination through the body (e.g., cooler light sources using fiber optic bundles so as to avoid the possibility of burning the patient) and improvements in infrared cameras for better detection. Scattering of the infrared radiation as it passes through body tissue, however, remains a key problem in obtaining clear images.
A number of attempts have been made to reduce or eliminate the effects of scattered light. See, for example, D. A. Benaron, "Measuring and Imaging in Tissue Using Near-IR Light," Optics & Photonics News, (October 1992), p. 27, and S. Svanberg, "Optical Tissue Diagnostics: Fluorescence and Transillumination Imaging," Optics & Photonics News, (October 1992), p. 31, and references cited therein. One approach to the problem makes use of so-called time-resolved methods. Time-resolved methods discriminate photons of light received at the detector based on the time taken to transit the sample. In one form of time-resolved method all the photons traversing the sample are detected and their arrival times are recorded. The image is then mathematically reconstructed from the photon delays as they travel from the emitter to the detector. In another method an attempt is made to receive only the so-called ballistic photons, which are those that traverse the sample with no scattering. In the absence of scattering, the photon path is linear and conventional radiological analysis may be used to view and interpret the image. This method, sometimes referred to as time-gating, has the advantage that it avoids the computationally complex image reconstruction called for in the more general time-resolved method where all photons are detected and used to construct the image. Time-gating is based on the concept that light which exits a transilluminated sample earlier has traveled a shorter and straighter path in the sample than light exiting later. The earlier light has undergone fewer scattering events and thus contains more information about the spatial localization of absorption within the sample. Time-gating methods then seek to detect only the early-arriving light and to block out the later-arriving light. The problem with these methods is that in a strong scattering environment there are not many ballistic photons to be detected. This places demanding requirements on the detector and often calls for averaging techniques to improve the signal-to-noise characteristics which can wash out information about the image.
Time-gating has been applied in various contexts. For time-gating studies applied specifically to imaging of tissue, see for example the work of S. Andersson-Engels et al., "Time-resolved transillumination for medical diagnostics," Optics Letters, Vol. 15 (November 1990), p. 1179, or M. R. Hee et al., "Femtosecond transillumination optical coherence tomography," Optics Letters, Vol. 18 (June 1993), p. 950, or L. L. Kalpaxis et al., "Three-dimensional temporal image reconstruction of an object hidden in highly scattering media by time-gated optical tomography," Optics Letters, Vol. 18 (October 1993), p. 1691.
Another technique which has been investigated in other contexts for processing images is known as optical upconversion. According to this technique the object under investigation is illuminated with radiation at a first frequency and the image beam carrying the image information is converted to a higher frequency at which it is more amenable to detection and processing. Extensive studies of upconversion are reported by A. H. Firester in a series of articles, see A. H. Firester, "Image Upconversion: Part III," Journal of Applied Physics, Vol. 41 (February 1970), p. 703, which cites the earlier articles in the series.
In a recent article M. Bashkansky and J. Reintjes report on a time-gated transillumination system that also has the potential for upconversion or downconversion of the image to more convenient wavelengths. M. Bashkansky and J. Reintjes, "Nonlinearoptical field cross-correlation techniques for medical imaging with lasers," Applied Optics, Vol. 32 (July 1993), p. 3842. The Bashkansky and Reintjes system has the shortcoming that it is inherently one-dimensional. To obtain an image, which is necessarily two-dimensional, the object must be scanned with a plurality of one-dimensional elements, and the image is constructed one line at a time.
Many of the time-resolved optical transillumination systems reported in the literature rely on wavelengths in the visible region of the spectrum. For transillumination of living tissue, however, there is a useful window of wavelengths between about 600 nanometers (nm) and about 1300 nm. At the 600 nm end of the range blood absorption falls off strongly and at the 1300 nm end of the range water absorption increases rapidly. It has been observed that within this range, however, tissue absorbance is comparatively low, permitting a greater transmitted light signal to be detected. For the longer wavelengths above about 1000 nm scattering effects are also less pronounced, leading to more ballistic photons. Nevertheless, good tissue transillumination imaging schemes are not available in this spectral range because of the lack of detectors with good sensitivity and low noise in this spectral region and because many of the reported transillumination schemes are not particularly suited to this wavelength range.